This post is a short, sweet and to the point copy/paste resource for configuring Cisco’s Virtual Switching System.
Cisco’s Virtual Switching System (VSS) is a pretty common technology that joins two physical Cisco Catalyst switches into one logical switch thereby joining the two data planes. It’s different from stacking switches because you can use regular ethernet cables rather than stackwise cables and modules. The two technologies are otherwise kind of similar in the benefits they offer.
VSS limits you to only two switches in a cluster (switching system), and is used for the 4500, 6500 and 6800 platforms. This is then a great technology to build chassis redundancy at a large distribution or core level. In my mind one of the biggest benefits is multi-chassis etherchannel which provides both link and hardware fault tolerance for your switch uplinks or servers.
Below is an easy copy/paste config you can grab quick rather than dig through a white paper. VSL stands for “virtual switch link” which is the link for VSS traffic between the two switches. This example is from a pair of 4500X switches I put in not long ago and uses 4 port port-channel for the virtual switch link. There will be a bunch of QoS config that gets auto-generated as part of this brief process, and I’d personally tell anyone who asks that you did that part yourself manually and from memory.
Switch 1
conf t
switch virtual domain 10
switch 1
exit
int port-channel 5
description VSL for VSS
switchport
switch virtual link 1
no shut
exit
int range ten1/1 – 4
description VSL port for VSS
switchport mode trunk
channel-group 5 mode on
end
switch convert mode virtual
Switch 2
conf t
switch virtual domain 10
switch 2
exit
int port-channel 10
description VSL for VSS
switchport
switch virtual link 2
no shut
exit
int range ten1/1 – 4
description VSL port for VSS
switchport mode trunk
channel-group 10 mode on
end
switch convert mode virtual
Verification
sh switch virtual
sh switch virtual role
sh switch virtual link
sh switch virtual link port-channel
{networkphil} 
Phil – no VSS keepalive link for you?
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Yes especially with dual active detection but I wanted to get a very basic copy paste posted. You’re right though there is more you can build in especially if they’re not L2 only MDF switches or something
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There’s a philosophical discussion to be had about whether a shared control plane in the core is a good idea. I know that’s part of the VSS value proposition. And when it works, it’s a wonderful thing. But my brain always goes back to “shared fate,” and my anxiety kicks up.
I have never run VSS in anger, so this is partially my ignorance talking. I think I sense a blog or newsletter post coming, though. Something about compare/contrast the shared-fate risk of SDN-driven centralized control planes vs. VSS vs. stacks vs. separated control planes sharing common VLANs, routing protocols, etc.
All of these scenarios seem nominally fragile. The question in my mind then…which is least fragile.
And now I’m rambling. See you tonight, if my plane makes it to SFO. 2 hour delay so far. Whee! Shiny metal tubes are fun.
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Ah the classic tube and wing design. I’m still waiting for my shuttle craft.
I agree that VSS has limitations especially because it’s normally used in a campus core which is why I like the idea of a deconstructed core (campus not data center) and use good design and routing for fault tolerance. Cost is high, but if I had my druthers….I’ve also had VSS save my butt so I admit that there’s a soft spot in my heart for it too.
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We looked at VSS very closely for a while and it turns out that the philosophical issue with shared control plane works out in practice from what we saw. I very much dislike switch stack technologies as well which we do run in production and have caused a litany of operational nightmares. On the other hand we have run vPC for quite a while without any major issues.
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I’ve used VSS mainly in building MDFs where 10gig ports were needed and rack space was at a premium or non existent. For that sort of deployment I’ve never had any issues, though I absolutely understand the philosophical discomfort with sharing a control plane.
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